Low-divergence and low-sidelobe orbital angular momentum generation enabled by a modulated metasurface.

Abstract

In this Letter, a low-divergence, low-sidelobe circularly polarized (CP) vortex emitter is proposed to generate a high-gain orbital angular momentum (OAM) beam of mode +l. Its leaky-wave radiation arises from the interaction between a bound surface wave and the spatially varying patches. A direct aperture field synthesis method is developed based on the Hankel transform relation inherent in the topological nature of vortex waves. To manipulate the aperture-field amplitude and phase simultaneously, a nonuniformly modulated metasurface (MTS) is utilized, and the modulation parameters are determined by solving local periodic problems. Modulated MTS emitters with different OAM modes are designed and simulated, all exhibiting low divergence and high sidelobe suppression ratios (SLSRs). To validate the proposed method, an emitter with OAM mode l = +1 is fabricated and tested. Experimental results demonstrate a divergence angle of 4°, an SLSR of 25.6 dB, and a gain of 23.3 dBic at 17 GHz, while the emitter maintains a thickness of 0.043λ0 and an overall profile of 0.663λ0. The proposed emitter is well suited for vortex imaging.